When an encoded audio signal is transmitted over a wired/wireless network, if partial packets are damaged or distorted due to a transmission error, an erasure may occur in partial frames of a decoded audio signal. If the erasure is not properly corrected, sound quality of the decoded audio signal may be degraded in a duration including a frame in which the error has occurred and an adjacent frame.
Regarding audio signal encoding, it is known that a method of performing time-frequency transform processing on a specific signal and then performing a compression process in a frequency domain provides good reconstructed sound quality. In the time-frequency transform processing, a modified discrete cosine transform (MDCT) is widely used. In this case, for audio signal decoding, the frequency domain signal is transformed to a time domain signal using inverse MDCT (IMDCT), and overlap and add (OLA) processing may be performed for the time domain signal. In the OLA processing, if an error occurs in a current frame, a next frame may also be influenced. In particular, a final time domain signal is generated by adding an aliasing component between a previous frame and a subsequent frame to an overlapping part in the time domain signal, and if an error occurs, an accurate aliasing component does not exist, and thus, noise may occur, thereby resulting in considerable deterioration of reconstructed sound quality.
When an audio signal is encoded and decoded using the time-frequency transform processing, in a regression analysis method for obtaining a parameter of an erasure frame by regression-analyzing a parameter of a previous good frame (PGF) from among methods for concealing an erased frame, concealment is possible by somewhat considering original energy for the erased frame, but an error concealment efficiency may be degraded in a portion where a signal is gradually increasing or is severely fluctuated. In addition, the regression analysis method tends to cause an increase in complexity when the number of types of parameters to be applied increases. In a repetition method for restoring a signal in an erased frame by repeatedly reproducing a PGF of the erased frame, it may be difficult to minimize deterioration of reconstructed sound quality due to a characteristic of the OLA processing. An interpolation method for predicting a parameter of an erased frame by interpolating parameters of a PGF and a next good frame (NGF) needs an additional delay of one frame, and thus, it is not proper to employ the interpolation method in a communication codec sensitive to a delay.
Thus, when an audio signal is encoded and decoded using the time-frequency transform processing, there is a need of a method for concealing an erased frame without an additional time delay or an excessive increase in complexity to minimize deterioration of reconstructed sound quality due to packet losses.